1. Field of the Invention
This invention relates generally to an enhancing circuit which is used for the image processings of TVs, VTRs, or the like, and particularly to a circuit arrangement for enhancing a color difference signal of a video signal by utilizing the change of a luminance signal of the video signal.
2. Prior Art
As is well known in the art, there are circuits for enhancing a luminance signal. If these circuits are applied to a circuit for enhancing a color diffence signal, the ringing of colors occurrs in every color-changing portions of images. In the signal processing method of home VTRs, a color difference signal is converted into a low-band signal by an AM recording method in order to reduce the undesirable effect of jitter. Since the frequency-converted color difference signals are of low frequency, i.e. several hundreds of kilohertz (for example, 629 Hz for VHS format and 688 kHz for Betamax format), the degree of azimuth loss is negligibly low thereby causing crosstalk between adjacent tracks on a recording medium. In addition, the AM recording method has disadvantages such as an unsatisfactory SN ratio and a narrow frequency band. Although the SN ratio has been improved by a noise reduction circuit using a field memory in recent years, there is a problem of color-ringing originated from the narrow frequency band. Besides, although there is a home VTR whose frequency band of a luminance signal is wider, since the same color difference signal processing method as previous VTRs is used in the home VTR, the color-ringing problem still remains as a large drawback.
Hence, the enhancement of the color difference signal is considered. One example of the conventional enhancing circuit is disclosed in "Picture Signal Improvement in Color. TV Receivers", IEEE, CE31, No. 3, August 1985. In this enhancing circuit, however, since only information in the color difference signal is used for the enhancement, satisfactory enhanced color difference signal is hardly produced because the frequency band is narrow, i.e. information is not enough. Accordingly, the deterioration of SN ratio cannot be satisfactorily improved.